Late years, along with popularization of displays, a level of performance required for an optical film for use therein has become increasingly higher. In order to meet the requirement, there is a need for developing an optical film by utilizing a new material and a new process.
As one type of optical film, there is a retardation film. Properties of the retardation film include a wavelength dependence of a retardation value, and a type having a property in which a retardation value becomes larger as measured by using light having a longer wavelength (this property will hereinafter be also referred to as “reverse wavelength dispersion property”) has been developed.
Japanese Patent Application No. 2010-271860 discloses a technique of improving brittleness of a retardation film, particularly, a retardation film with a reverse wavelength dispersion property, from the standpoint of runnability, processability, product reliability, etc, in order to produce a large number of a wide and thin film, or the like.
JP 2009-086651A discloses a technique intended to, by using a retardation film which has a low melt viscosity and a low environmental load and exhibits excellent flowability, formability, heat resistance, long term stability in retardation value, and wavelength dispersion property, provide a circularly polarizing plate and a display device capable of creating high contract even when viewed obliquely and realizing good display without becoming bluish.
However, in these literatures, no discussion is made about display unevenness due to a phenomenon that a retardation variation of a retardation film used, e.g., in a polarizing plate for a display device, varies depending on position in the retardation film, in a situation where the retardation film is exposed to high-temperature conditions, e.g., during use of the display device, over a long period of time.